Globally, human modification of land and the human-aided introduction of exotic species are considered to be the main drivers behind species decline and extinction. Human colonisation of New Zealand and subsequent development resulted in the conversion of native forests and shrublands into productive land, which together with the introduction of invasive species has caused the decline of many species, and many more are now threatened with extinction. This issue is particularly prevalent in the lowland eastern areas of the South Island, in which there has been intensive land development. One strongly affected group are species within the genus Olearia, in which several species are threatened with extinction, including nationally declining Olearia lineata. In contrast, other species in this genus, such as non-threatened O. bullata, are seemingly not as strongly impacted. My study investigated the mechanisms behind why O. lineata is nationally declining in comparison to the non-threatened congener O. bullata. To do this, I investigated two main research questions; one question investigated the demographic structure of populations to determine if regeneration is occurring and the vegetation composition of the surrounding community. Sampled populations indicated regeneration failure in O. lineata populations but also in O. bullata populations, despite its non-threatened status. I found that a high proportion communities contained exotic grasses, therefore, the second question was how the presence of the exotic grass- Agrostis capillaris, affected recruitment of O. lineata and O. bullata. Two glasshouse experiments investigated how the presence or absence of grass affected germination and growth of seedlings. Both Olearia species were found to germinate in the presence of grass, but seedlings grew significantly better in the absence of grass. Olearia are also important native Lepidoptera hosts as a suite of native moths feed exclusively on this genus. Therefore, a third research question investigated the abundance of larvae and the community composition of Lepidoptera on O. lineata and O. bullata in comparison to another Lepidoptera host Coprosma propinqua. Moth larvae were collected and some species successfully reared to find there was higher larvae abundance on O. lineata and high species overlap between O. lineata and O. bullata with little species overlap between the two Olearia species and C. propinqua. Overall, my research found that exotic invasive grasses potentially cause regeneration failure in both O. bullata and O. lineata. However, the largest impact in the difference of threat status is due to O. lineata populations occurring in the most developed lowland areas of the eastern South Island, whereas O. bullata populations occur in less human modified, higher altitude areas. The further development of these areas through human activity threatens the future persistence of these Olearia species and the future of the Olearia Lepidoptera specialists.

The main objective of this research was to determine the seed bank longevity of Melinis repens at two Southern Florida sites. Seeds were divided among different exposure levels (shade versus sun) and depths (surface versus buried) and tested for baseline viability using 2,3,5-Triphenyl-tetrazolium chloride. Statistical analysis determined that at the pine rockland site there was a significant interaction between time, exposure, and depth. The initial mean viability at this site declined from 49.71% to 11.26% and 13.06% for sun/buried seeds and sun/surface seeds, respectively, by month 8. The mean viability of shade/surface seeds and shade/buried seeds declined to 24.56% and 22.06% after 8 months. There were no significant effects in the Florida scrub. In order for land managers to completely remove this species from a site, treatment with herbicide will need to continue for a minimum of one year to effectively kill all viable seeds in the seed bank.

Mycorrhizae have been used in restoration for decades. However, studies assessing the use of mycorrhizae in Bromus tectorum-invaded areas of the Great Basin are limited. Two greenhouse pot experiments were conducted to assess the role of mycorrhizae in sagebrush restoration. The first objective (Chapter 2) was to determine the response of Pseudoroegneria spicatum, Elymus elymoides, and B. tectorum to mycorrhizal symbiosis by altering phosphorus, density, species, presence of mycorrhizae and water levels in a 5 factor design. To assess the mycorrhizal response, a variety of morphological and physiological traits were measured, such as tissue P concentration, specific root length, specific leaf area, carbon isotope discrimination, etc. The effects of the different treatment combinations were analyzed using ANOVA. The second objective (Chapter 3) was to determine the role of different inocula in competition between the three grasses. Species, density, and inoculum type were altered in a 3 factor design. Inoculum was cultured on Allium plants. The effect of locally cultured inoculum on the species was compared to the effect of commercial inoculum. The response of each species to mycorrhizae with different species compositions and densities was assessed. Morphological measurements were used to determine each species response to the different factor combinations. The effects of the different treatment combinations were analyzed using ANOVA. This research provides land managers with information regarding the efficacy of using local versus commercial inocula and whether they should use mycorrhizae in restoring their systems.

Kangaroo rats (Dipodomys spp.) are frequently characterized as keystone species for their role in altering soil characteristics, changing habitat structure through seed consumption and dispersal, and being important primary consumers in their ecosystem. They are arid adapted and known to forage in areas with sparse vegetation. Studies suggests densely vegetated habitat to be unsuitable for kangaroo rats because plants are an impediment to their locomotion and predator avoidance behaviors. This study focuses on an invasive grass, South African Veldt (Ehrharta calycina), that converts landscapes with sparse vegetation into dense grassland habitats, and the Lompoc kangaroo rat (Dipodomys heermanni arenae) that occupies some of those modified landscapes. I explored the proximate effects of Veldt grass by assessing the locomotion of D.h. arenae in three Veldt grass densities. I hypothesized that Veldt grass influences kangaroo rat locomotion, but that performance could also be influenced by experience with the grass. Kangaroo rats with long-term experience with Veldt grass (i.e., those occupying a habitat containing Veldt grass) and short-term experience (two-night habituation in an artificial Veldt grass patch) were tested by pursing the animals through runways of different grass densities and measuring the amount of time spent crossing the runway, amount of time spent stopped, average velocity, and amount of motivation required to initiate and sustain movement. I also monitored habitat use during the two-night habituation period in order to assess habitat utilization among three Veldt grass density habitat patches. I hypothesized that Veldt grass may influence normal habitat utilization patterns in D.h. arenae: specifically, the avoidance of the densest habitats and preference or disproportionate utilization of the most open habitat. I found, when the animals were left alone to forage and explore, they spent significantly more time in habitat patches containing Veldt grass than in a control patch containing zero percent cover. However, in locomotion trials, Veldt grass had a negative effect on locomotory performance. These effects seem to scale with grass density, and were ameliorated to some degree by familiarization: animals from a Veldt grass habitat of origin performed better in novel Veldt grass templates than animals from a non-Veldt habitat of origin; however, both groups performed equally well after two nights’ habituation to the templates. These results suggest that learning occurred in two nights and that it increased the kangaroo rats’ ability to locomote through the grass when pursued. I note that performance studies often do not take into account the amount of motivation employed to initiate and sustain running of the test animals, and suggest that this be considered in future studies. Furthermore, the learning capacity of a kangaroo rat, as well as a community level perspective that considers neutral or even positive trophic interactions among natives and invasives, must be considered in conservation and management decisions in the future.